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Jeyun Yeom, Giacomo Lorenzin, Lea Ghisalberti, Claudia Cancellieri, Jolanta Janczak-Rusch

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Microsoft Word - AMO_TSTA_A_2357536.docxFull Terms & Conditions of access and use can be found athttps://www.tandfonline.com/action/journalInformation?journalCode=tsta20Science and Technology of Advanced MaterialsISSN: (Print) (Online) Journal homepage: www.tandfonline.com/journals/tsta20The thermal stability and degradation mechanismof Cu/Mo nanomultilayersJeyun Yeom, Giacomo Lorenzin, Lea Ghisalberti, Claudia Cancellieri & JolantaJanczak-RuschTo cite this article: Jeyun Yeom, Giacomo Lorenzin, Lea Ghisalberti, Claudia Cancellieri &Jolanta Janczak-Rusch (22 May 2024): The thermal stability and degradation mechanismof Cu/Mo nanomultilayers, Science and Technology of Advanced Materials, DOI:10.1080/14686996.2024.2357536To link to this article:  https://doi.org/10.1080/14686996.2024.2357536© 2024 The Author(s). Published by NationalInstitute for Materials Science in partnershipwith Taylor & Francis Group.Accepted author version posted online: 22May 2024.Submit your article to this journal View related articles View Crossmark datahttps://www.tandfonline.com/action/journalInformation?journalCode=tsta20https://www.tandfonline.com/journals/tsta20?src=pdfhttps://www.tandfonline.com/action/showCitFormats?doi=10.1080/14686996.2024.2357536https://doi.org/10.1080/14686996.2024.2357536https://www.tandfonline.com/action/authorSubmission?journalCode=tsta20&show=instructions&src=pdfhttps://www.tandfonline.com/action/authorSubmission?journalCode=tsta20&show=instructions&src=pdfhttps://www.tandfonline.com/doi/mlt/10.1080/14686996.2024.2357536?src=pdfhttps://www.tandfonline.com/doi/mlt/10.1080/14686996.2024.2357536?src=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1080/14686996.2024.2357536&domain=pdf&date_stamp=22 May 2024http://crossmark.crossref.org/dialog/?doi=10.1080/14686996.2024.2357536&domain=pdf&date_stamp=22 May 2024ACCEPTED MANUSCRIPT1 The thermal stability and degradation mechanism of Cu/Mo nanomultilayers Jeyun Yeom*, Giacomo Lorenzin, Lea Ghisalberti, Claudia Cancellieri, Jolanta Janczak-Rusch* Empa, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for Joining Technologies and Corrosion, Überlandstrasse 129, 8600 Dübendorf, Switzerland Abstract The microstructural evolution of Cu/Mo nanomultilayers upon annealing was investigated by X-ray diffraction and transmission electron microscopy. The isothermal annealing process in the temperature ranges of 300 – 850 ºC was conducted to understand the thermal behavior of the sample and follow the transformation into a nanocomposite. Annealing at 600 ºC led to the initiation of grain grooving in the investigated nanomultilayer, and it degraded into a spheroidized nanocomposite structure at 800 ºC. The sample kept the as-deposited Cu {111}//Mo{110} fiber texture up to 850 ºC. The residual stress was investigated to explain microstructure changes. The activation energy of degradation kinetics of Cu/Mo nanomultilayers was determined to understand the rate-determining mechanism for the degradation of nanolaminate structures. List of Keywords: Cu/Mo nanomultilayers, Annealing, X-ray diffraction, Fiber texture, Magnetron sputtering;  *jeyun.yeom@empa.ch, jolanta.janczak-rusch@empa.chPublisher: Taylor & Francis & The Author(s). Published by National Institute for MaterialsScience in partnership with Taylor & Francis Group.Journal: Science and Technology of Advanced MaterialsDOI: 10.1080/14686996.2024.2357536https://crossmark.crossref.org/dialog/?doi=10.1080/14686996.2024.2357536&domain=pdfACCEPTED MANUSCRIPT2   1. Introduction Nanomultilayers (NMLs), also called nanolaminates, or nanolamellars, are systems characterized by the repetitions of numerous layers where the thickness of each layer varies between 1 and 100 nm. The presence of a large number of interfaces, together with the possibility to combine complementary materials, can lead to a wide range of novel and outstanding properties, which differ to a great extent from those observed in monolithic films [1–4]. NMLs have been successfully produced using various methods, including accumulative roll bonding [5,6], chemical vapor deposition [7,8], electrodeposition [9,10], and physical vapor deposition (PVD) [11,12]. Among these techniques, magnetron sputtering, a form of PVD, offers distinct advantages in the synthesis of NMLs. Indeed, it enables the fabrication of a wider range of material systems as nanocrystalline configurations while providing precise control over crucial characteristics such as the number of interfaces, grain structure, and layer thickness.  The improved performances at smaller size scales exhibited by copper thin films have garnered significant interest in both fundamental science and applied research, in particular, aimed at the improvement of the reliability of microdevices [13–16]. In order to enhance the material properties of copper, extensive research has been carried out to develop advanced copper-based binary alloys, composites and multilayer systems. Specifically, notable investigations have focused on cobalt-copper (Co-Cu) [17–19], copper-tungsten (Cu-W) [20], copper-tantalum (Cu-Ta) [21], copper-chromium (Cu-Cr) [22], and copper-molybdenum (Cu-Mo) material systems [23].   In this study, we focus on Cu/Mo nanoscale metallic multilayers produced via magnetron sputtering. This system has been selected for the following reasons: a) Cu and Mo exhibit immiscibility up to relatively high temperatures. The solid solubility between bulk Cu and Mo is extremely low, at less than 1.5 wt %, even up to 900 °C. As a result, annealing at moderate temperatures below 900 °C can prevent the formation of Cu-Mo alloys [24].  b) Cu/Mo multilayer structure represents a promising system for thermal management thanks to the combination of excellent thermal conductivity (Cu) and low coefficient of thermal expansion (Mo). Cu-Mo-based materials have found numerous applications for their ability to collect and dissipate heat in fields such as high-power semiconductor devices, heat sinks, electronics, and electrical engineering [25,26]. It was previously shown that these multilayers have a very low electrical resistivity [27,28].  ACCEPTED MANUSCRIPT3  The numerous applications demonstrate the wide-ranging utility of Cu-Mo NMLs in areas that require excellent thermal management, and high thermal and electronic conductivity. However, the high Gibbs energy resulting from the high density of interfaces poses a risk to the microstructural stability of the multilayered structure. For this reason, understanding the microstructural changes upon annealing of such laminated materials is of paramount importance, as it directly affects their performance and reliability at elevated temperatures. In this work, the microstructure evolution of Cu/Mo NMLs upon annealing up to 850°C is investigated. Texture, morphology, and residual stresses are analyzed after different heating stages. X-ray diffraction (XRD) and in-situ high temperature (HT) XRD experiments were carried out to understand texture evolution, interface structure changes upon annealing, and the kinetics in the underlying mechanism of NMLs degradation.  2. Experimental Procedure  Cu/Mo NMLs were deposited by DC magnetron sputtering on Si (001) substrates with amorphous silicon nitride on top. The thickness of the silicon nitride layer was 90 nm and it served as a barrier layer to prevent Cu diffusion into the Si substrate during the annealing process.  Substrates were cleaned with acetone, ethanol, and isopropanol. Each cleaning step was 3 minutes long and was followed by Ar drying. RF cleaning process was carried out with 50 W power at 0.2 mbar (15 mTorr) of Ar-pressure for 2 minutes. DC magnetron sputtering in a high vacuum chamber (base pressure ≤ 10−8 mbar) was used for the deposition of Cu/Mo with gun power 80 W and Ar pressure 0.027 mbar (2 mTorr). The bilayer structure Cu10nm-Mo10nm was repeated 10 times to make a total of 200 nm thick layers. (See Fig. 1a) Isothermal annealing was carried out at temperatures 300 °C, 600 °C, 800 °C, and 850 °C for 100 minutes in a high vacuum environment with pressure below 10-5 mbar. The heating rate used to arrive at the pre-defined annealing temperature was 20 K/min. Transmission electron microscopy (TEM) lamellas were prepared with an FEI Helios 660 Nanolab Dual-Beam Focused Ion beam/Scanning electron microscopy (FIB/SEM) system with a 30 kV Ga-ion beam. To remove the damaged layer, the cleaning process with 5 kV and 2 kV Ga-ion beams was carried out. TEM and STEM were performed using JEOL2200FS equipped with an Energy-dispersive X-ray spectroscopy (EDX) detector which was operated at 200 kV to characterize the microstructures of as-prepared samples.  A Bruker D8 Discover X-ray diffractometer was used to perform θ-2θ scans and to measure the texture of samples at room temperature and after the annealing process at 850 °C. To assess the texture, pole figures were acquired around the (111) reflection of Cu and the (110) reflection of Mo. Diffraction patterns were recorded using Cu target at 40 kV and 40 mA. A modified crystallite group method introduced by Tanaka et al. was used for measuring residual stress in a ACCEPTED MANUSCRIPT sampstressof respreseIn-sPROMequipmixtuml/mnm, a700, a 3. R3.1 MFigurcrossright- Asillustrstructshowbetweimageple with a strses of texturesidual stress ented in the situ high-temMPD X-raypped for heaure of 5 volmin. Diffractioat 40 kV andand 725 º C Results andMicrostructre 1. As-pr-sectional im-hand side, ds-prepared Curated in Fig.ture. The graws the Brighteen the nanoe, which shoong fiber texed Cu/Mo Nin NMLs arstudy [30].  mperature Xy diffractomating and gal.% H2 (99.on patterns (d 40 mA) wefor a total isod discussionture and texrepared sammage with ovd) pole figureu/Mo NMLs 1a. Fig. 1b ain-like struct Field (BF)olayers can bows a resolvexture [29]. BNMLs were are equibiaxiaXRD experimmeter with as feeding (5999%, Mess(2θ range of ere recorded othermal holn xture of as-mple: a) illverlaid EDX es of Cu {111s have a Cu1displays thectures were fo)-STEM crosbe recognizeed Cu/Mo NM4 Based on the analyzed undal and using ments were a gas-tight 5850 TR, Brser) in N2 (f 37-46º; usinevery ~ 5 mlding time of-prepared Clustration, bimage; the c1} and Mo {10nm-Mo10ne SEM surfacformed uniforss-sectional ed, as confirmML structuremethodologyder the assumthe selectioncarried outAnton Paarrooks instrum(99.999%, Mng Cu Kα1,2 rminutes duringf 5 hours. Cu/Mo NMLb) SEM sucolors of the110}. nm bilayer stce image of trmly withouimage of thmed also bye. y of Tanaka mption that an criteria of t with a PAXRK-900 ment), applyMesser) at a radiation, 𝜆g isothermalLs  urface image elements artructure withthe as-prepart any cracks he sample. Hy the overlayet al., the reall the compof diffraction pANalytical Xheating chaying an H2/Nflow rate o = 0.l annealing age, c) BF-Sre indicated oh 10 repetitiored Cu/Mo Nand voids. FHere, the intyed EDX maesidual onents planes X'Pert amber N2 gas of 100 15418 at 675, STEM on the ons, as NMLs Fig. 1c erface apping ACCEPTED MANUSCRIPT Thand spolycstructand CMatmagnand itIn thiThe afiber an ouin-plaintenline windicaFigur850 °peaks 3.2 SThesuperhe deposited smooth. By crystalline grtures have beCu/Nb [30]. terials propenetic propertits changes isis regard, Figanalysis revetexture. Theut-of-plane tane random sity. The tiltwith the {11ate a peak orre 2. XRD θ°C. The intens from non-mStructure a results of θrlattice strucnanolaminatfurther incrain-like strueen also obserties such ies [37], stros crucial for g. 1d shows eals a prefere red point atexture with crystallograting angles fo11} and {11riginating fro-2θ scans ofnsities were monochromaanalysis by Xθ-2θ scans ofcture from ate structure reasing the uctures, asviserved in othas thermal ongly dependperforming rthe pole figurential orientt the center Cu {111} //aphic orientor Cu and M0} textures.om the Si subf NMLs as-pplotted on atic Cu radiatXRD beforef NMLs ranga multilayer 5 was initiallythickness, tsible in the sher NML systconductivityd on the texturesidual stresures of the Ctation in the of both pole/ Mo {110} tations resulMo are measu The two bbstrate. prepared anda logarithmiction. e and after ging from 30contributesy, i.e., close the structuresurface imagtems, such ay [35], mecure of materiss analysis uCu {111} andlayer, i.e., Ce figures showorientation. lts in uniforured to be 70lack arrows annealed atc scale. Asterheat treatm0° to 50° are to the XRto the substre becomes wge (see Fig. as Cu/W [31–chanical proials. Understusing X-ray dd Mo {110} fCu {111} // Mws a high inAdditionallyrm ring stru0.53° and 60in the Cu 300 °C, 600risks in the fment e given in FiRD diffractiotrate interfacwavy by fo1b). Similar –33], Cu/Mooperties [36]tanding the tediffraction (Xfamilies of pMo {110}, wntensity, indiy, the presenuctures with0°, respective{111} pole 0 °C, 800 °Cfigure indicaig. 2. An arton profile me, flat rming wavy o [34], ], and exture XRD). planes. with a cating nce of h high ely, in figure C, and ate the tificial making ACCEPTED MANUSCRIPT6  satellite peak [38]. The satellite peak is evident in the as-deposited NMLs and it remains till 600°C. The contribution from the superlattice structure depends on the quality of the interface structure and the disorder of the intralayer structure [39] . These satellite structures disappeared and the Full Width at Half Maximum (FWHM)s of both Cu (111) and Mo (110) peaks significantly decreased after the annealing process over 800 °C. The results indicate the destruction of the periodic multilayer and the transformation into a nanocomposite (NC). The disappearance of satellite peaks in diffraction pattern and the grain grooving upon annealing has been already observed in immiscible multilayer systems at T > 700 °C [40].   3.3 Microstructure and texture evolution of Cu/Mo NMLs after annealing  SEM surface images of Cu/Mo NMLs structure after annealing at 300, 600, 800, and 850 °C are shown in Figs. 3a, c, e, and g, respectively. The surface structure did not undergo significant modifications until annealing up to 600 °C. After annealing at 800 and 850 °C the grain-like structure grew and void formation was observed in the surface structure.  BF-STEM cross-sectional images after annealing with overlapping EDX images are shown in Figs. 3b, d, f, h. Fig. 3b indicates that after annealing at 300 °C in-plane grain growth occurred and NMLs structure was conserved. Initiation of grain grooving was observed with the alignment of grain boundaries after annealing at 600 °C and significant in-plane grain growth was verified as shown in Fig. 3d. Dotted lines in Fig. 3d indicate aligned grain boundaries, which make a stair-like grain boundary structure. (see Fig. 6a)  Grain boundary grooving is known to initiate the degradation and pinch-off of multilayer structures with immiscible constituents leading to rapid spheroidization of the discontinuous layers [41–43]. Cu/Mo NMLs in the present work degraded to nanocomposite structure after annealing at 800 °C as shown in Fig. 3f, though there are studies reporting morphologies changes, e.g, zig-zag layered structures in Cu/Nb, multilayer structures after annealing [12,42,44,45]. The partly delamination of NC structure from amorphous silicon nitride is shown in Fig. 3f. Skeleton structure with nanocomposite and re-bonding of NC structure to substrate were observed in Fig. 3h. The void formation might result from re-bonding of NC to substrate.  The factors influencing the formation of zigzag and pinched-off microstructures in annealed NMLs were elucidated through model calculations based on the results of Cu/Mo, Cu/Ag, and Cu/Nb systems [42]. According to the model [42], the final microstructure after annealing is determined by the aspect ratio of grain dimensions and the ratio of the distance between the two nearest triple junctions to the in-plane grain size. The microstructure of Cu5nm/W5nm NMLs film with 100 bilayer repetitions was examined previously and similar trends were observed: the NMLs structure completely degraded into NC structure at the same annealing temperature [40].  ACCEPTED MANUSCRIPT7  Fig. 3i indicates that the crystallographic orientation of Cu/Mo was conserved exhibiting Cu {111} // Mo {110} fiber texture even though NMLs structure degraded into NC structure with voids at 850 °C. This result ensures that the methodology chosen for measuring residual stress can be used for all the samples [30].    Figure 3. Microstructure and texture evolution of Cu/Nb NML: a), c), e), g) SEM surface images of Cu/Mo after annealing; b), d), f), g) BF-STEM cross-sectional images of with overlaid EDX images (the color of elements was kept as Figure 1); i) pole figures of Cu {111} and Mo {110} after annealing at 850 °C. The dotted lines in d) indicate aligned grain boundaries.   3.4 Residual stress analysis Fig. 4 shows the residual stress evolution of Cu/Mo nanomultilayer. As-prepared NML exhibited compressive stress in both Cu and Mo layers [30]. During the annealing process, thermally induced stresses develop in the film, due to the coefficient of thermal expansion (CTE) mismatch between substrate and film material. During the initial stage of a heating or cooling segment, when the deformation is purely elastic, the stress-temperature behavior of the film follows the thermoelastic line given by  𝜎  = 𝜎 +  (𝛼 − 𝛼 )(𝑇 − 𝑇 )                (1)  where σ0 is the stress at the initial (room) temperature T0, Efilm and νfilm are Young's modulus and Poisson's ratio of the film, Efilm/(1-vfilm) is the biaxial elastic modulus of the film, ACCEPTED MANUSCRIPT8  and αsub and αfilm are the CTE of substrate and film respectively [46]. In this work, σ0 measured at room temperature was initially generated during deposition and it was modified due to the temperature difference between the annealing temperature and room temperature. The measured stress of σ0 by XRD can be divided into coherence stress and deposition stress [47] and it was found to be compressive for the as-deposited NML. After the annealing process, residual stress measured ex-situ turned into tensile stress (see Fig. 4). Both Cu and Mo exhibited tensile stress after annealing at T > 300°C with a maximum tensile value between 600-800 °C, as measured by ex-situ XRD. Above these temperature ranges, stress relaxation occurs in both layers. To explain this turnover of stress, one has to analyze what happens during the thermal treatment. During the heating step, thermally induced compressive stress will be applied to NMLs as expected from Eqn. 1 (αcu =16.6 × 10−6  K−1, αMo = 5 × 10−6  K−1, αSi = 2.6 × 10−6 K−1 [48]). At high temperatures, residual stresses in both Cu and Mo layers become compressive as the lattice expands following the thermal expansion (αfilm > αSi). Then, the compressive residual stresses in both layers can be relaxed through diffusion, creep, or dislocation climb and glide [46,49]. During cooling down, tensile stress builds up in the NMLs. Tensile stress is indeed measured by XRD at room temperature, after annealing. The delamination of Cu/Mo NMLs occurred mainly near the edge of the substrate after 800 °C annealing. This is not surprising since the vulnerability of film edge to delamination was reported [50,51]. The delamination can be explained by high residual stresses in the Cu nanolayers (1.3 GPa even after annealing at 600°C) [52]. The observed delamination of the Cu/Mo nanomultilayer from the Si substrate at annealing temperatures above 800°C, led to new pathways for the residual stress relief, resulting in a similar magnitude of residual stress in the annealed Cu and Mo layers as observed after annealing at 850°C. As previously mentioned, the sample surface morphology also transforms during the thermal treatment (See Fig. 3). Hillocks growth (i.e., protrusions of material on the surface) would be expected for this system as Cu hillock growth in Mo/Cu multilayer on Si substrate was already reported [48]. Moreover, several models were introduced to explain hillock growth, and a common factor in all of them was the requirement of compressive stress in the plane of a film [53]. However, there is no evidence of Cu hillock growth in the Cu/Mo NMLs samples in this work at any used annealing temperature, though a similar immiscible system of Cu/W NMLs on Si substrate exhibited important Cu hillock growth at about 500 °C annealing temperature [54]. The occurrence of Cu surface outflow may be influenced by the difference in magnitudes of initial compressive residual stresses. The compressive residual stresses of Cu and Mo in Cu/Mo are considerably lower than those of Cu and W in Cu/W [54]. Considerably high compressive growth stress on the hard W (>3 GPa in magnitude) compared to less than 0.5 GPa in Mo ACCEPTED MANUSCRIPT monosurfacFigur800 ° 3.5 GThe2coswhereenergthe raThowas oenergtempewheresummanneaPincexplabounddiscuEquaestimtheorolayers influce outflowinre 4 . Residu°C, and 850 °Grain groov morphology=     e θ is the ggy. The equaatio of  bough, grain gobserved in gies of the grerature of Me deeper gramarizes in a aling. The grch-off of NMained by refedary energy ussed, despiteation (2), andmated approxry (DFT), theuences the song upon anneal stress evo°C. a) residuving of Cu/y of grain gro (2)  grooving anation (2) assubecomes largrooving occuMo which wrain boundarMo is higherain boundaryschematic rrooving angleMLs can be pferences [42,of Cu/Mo Ne its importad the grain ximately 0.5e interface enofter Cu whealing [54]. lution of Cu/al stress of CMo NMLs ooving in equgle of the gumes isotropicge, the groovurred in bothwill lead to py can be scar than that oy grooving inrepresentatioes of Mo in tprevented by45]. The theNMLs. Howence in designboundary en ~ 2.0 J/mnergy of Cu/M9 hich adapts tu/Mo NML asCu. b) residuaand kineticuilibrium cangrain boundac interface enving angle beh cases of Cupinch-off as aled with the of Cu. A simin Ta was obon the morpthe Cu nanoly controllingeoretical preever, the intening stable Cnergy of Mom  at 600 ºCMo was calcto the W lays-prepared anal stress of Mcs of NC strun be expressary (see Fignergies and mecomes smallu, and Mo, deshown in Fimelting temmilar result wbserved leadphological chlaminate matin-plane graedictions neeerface energyCu/Mo NMLo [57], the iC. Recently,culated for foyer and relend annealed Mo.  ucture formed as followg. 5a), 𝛾  imechanical el leading to peeper grain big 5a. This ismperature [56was obtainedding to pinchhange of Cutrix are preseain size and ed interface y of Cu/Mo L structures. interface ene by using dour distinct steases its streat 300 °C, 6mation  ws:  is grain bouequilibrium [5pinch-off.   boundary gros because th6], and the md in Cu/Ta Nh-off [43]. Fu/Mo NMLs ented in Tabllayer thicknenergy and has been scBased on Taergy of Cu/Mdensity functructures, yieess by  600 °C, undary 55]. If     ooving he free melting NMLs ig. 5b upon le 1. ness as grain arcely able 1, Mo is ctional elding ACCEPTED MANUSCRIPT10  in a value range from 3.09 to 4.11 J/m  [58]. The experimentally estimated interface energy is smaller than the calculated value. Given that a sharp rise in interface energy is observed as the temperature decreases [59], this discrepancy can be explained by the fact that the Density functional theory (DFT) calculation assumes 0 K. As the superlattice structure of NMLs contributes to additional XRD peaks, the degradation of the multilayer structure and its kinetics upon annealing can be investigated by monitoring satellite peak intensity in-situ HT XRD [40,60]. When NML transforms into an NC, satellite peaks disappear as the layer periodicity along the z direction is lost. The ending results are single diffraction peaks of individual Mo and Cu components. Fig. 6a shows θ-2θ scans as a function of isothermal holding time, shown exemplarily for an annealing temperature fixed at 700 °C. Each profile was obtained every 5 min, changing the color from dark red to bright red. At the end of the isothermal treatment, independently of the temperature used, higher peak intensity of Cu (111) and Mo (110) reflections was observed. The satellite peaks, present at the beginning, gradually disappeared as annealing time elapsed. Fig. 6b shows the peak intensity change of the satellite peak indicated in Fig. 6a. The evolution of other satellite peak intensity follows a similar trend confirming the reliability of the method used. For the first ~2000 sec of the isothermal annealing, more pronounced for T < 725 °C, an increase of satellite peak intensity was observed. This was also reported in previous work [40], and it was attributed to a reduced roughness at the interfaces upon annealing. A lowered interface disorder increases the satellite peak intensity in a superlattice. After this stage, the overall satellite peak intensity begins to decrease linearly with time, indicating the onset of the multilayer degradation into NC. The activation energy of NMLs thermal degradation was calculated by linear regression analysis in the Arrhenius plot. (See Fig. 6c) The activation energy is found to be 277.24 ± 22.93 kJ/mol (2.87±0.23 eV). Table 2 summarizes reported activation energies for grain boundary, surface, and lattice self-diffusion of Cu, and Mo. The activation energy of degradation of NMLs matches well with grain boundary and surface diffusion of Mo. The diffusion of Cu is much faster than Mo due to a much lower melting temperature. Therefore, the rate-determining step for the degradation of Cu/Mo NMLs is the diffusion process of Mo along grain boundaries or phase boundaries. In the case of Cu/W multilayers it was also found that the activation energy was in the range of W surface and grain boundary diffusion [40]. Both results on immiscible metallic multilayers validate the theory that mobility of the higher melting point metal, W or Mo, along the grain and interfacial boundaries, is the rate-limiting mechanism for NML degradation.   ACCEPTED MANUSCRIPT Figurof de Table Groo TableCu, aDiffuGrainSurfaLattic*AcctransiFigurexemre 5. a) HR-Tgradation mee1. The groovving angle e 2. Reportedand Mo usion pathwan-boundary ace ce cording to a rition metals cre 6. In-situmplarily showTEM cross-sechanism of ving angle oθ115d activation ay Cu (k62 75-87211 reference[66]can be assumu HT-XRD rwn for an annsectional imaCu/Mo NMLf Mo grain b 5° 1energies forkJ/mol) 7 ], the activatmed to be 1/2results. a) θnealing temp11 age of Cu/ML. boundary in tθ  11° r grain bounRef. [61] [62] [64] tion energy fo2 to 2/3 of thθ-2θ scans aperature fixeMo NML annethe Cu nanolθ  130° ndary, surfaceMo (kJ/m188- 282203-264375-423for grain boune activation as a functioned at 700 °C ealed at 600 aminate matrθ  126° e, and latticemol) 2* ndary diffusienergy for lan of isotherm(only scans °C, b) illusttrix. θ  102° e self-diffusiRef. - [63] [65] ion of bcc attice diffusiormal holdingafter every  tration ion of  on.    g time 5 min ACCEPTED MANUSCRIPT12  are shown for clarity). The intensity is plotted on a logarithmic scale. A multi-peak fit procedure was applied to deconvolute the integrated intensity of the different satellite peaks. b) Normalized intensity of satellite (I/I0) as a function of annealing time for the different holding temperatures investigated (650 ~ 725 °C). The intensity is plotted on a linear scale. c) It shows an Arrhenius plot deduced from stage II. The activation energy was evaluated using the slope.  4. Summary and conclusion   The microstructure evolution of Cu/Mo NMLs was investigated upon vacuum annealing (300–850 °C). Annealing at 600 ºC led to the initiation of grain grooving in the NML, which then degraded into a spheroidized nanocomposite structure and partially delaminated from the substrate at 800 ºC. By annealing at 850 ºC, void structures were formed, most possibly due to re-bonding to the amorphous silicon nitride substrate. The prepared sample kept Cu {111}//Mo{110} fiber texture, up to annealing at 850 ºC. Residual stresses of Cu/Mo nanomultilayer were analyzed to explain the degradation mechanism. No evidence of Cu hillock growth was observed. The interface energy of Cu/Mo is estimated approximately 0.5 ~ 2.0 J/m  at 600 ºC. The activation energy of degradation of Cu/Mo nanomulilayers was determined as 277.24 ± 22.93 kJ/mol (2.87±0.23 eV), indicating that the rate-determining step for the degradation of NML is the diffusion process of Mo along grain boundaries or phase boundaries. These findings play a crucial role in enhancing our understanding of the thermal stability and degradation mechanism of Cu/Mo nanomultilayers, which pave the way to support the development of nanolayered materials with improved performance.   Acknowledgments  Electron Microscopic Center of EMPA and Transport at Nanoscale Interfaces of EMPA are acknowledged for supporting TEM sampling and observation. Giacomo Lorenzin acknowledges the Swiss National Science Foundation (SNSF) under Project No. 200021_192224 for financially supporting this research. Jolanta Janczak-Rusch and Lea Ghisalberti acknowledge the Swiss National Science Foundation (SNSF) under grant number: 200021E_209588. The authors are grateful to P. Robin, S. Lohde, T. 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Mag. 8 997–1001 [66]  Brown A M and Ashby M F 1980 Correlations for diffusion constants Acta Metall. 28 1085–101    ACCEPTED MANUSCRIPT18  [[Graphical Abstract]]    ACCEPTED MANUSCRIPT19 This study investigates the microstructural evolution of Cu/Mo nanomultilayers during vacuum annealing up to 850°C and provides important insights into their thermal stability and degradation mechanisms for development and application. Impact statement